Trevor,
I agree with Roy - you don't need to do lots of very long walks. I rarely do a 30+ mile walk between Hundreds, but I was fortunate to move to Cumbria 4 years ago and I find that the extra strength gained from walking in the fells has greatly improved my general walking stamina. So get out in the Brecons as often as you can.
What Roy doesn't say is that the Hundred he has just completed is his 14th - there is nothing like experience to help you learn how to overcome the inevitable problems you encounter on a very long walk.
Ken Falconer put together some useful tips on the subject and his article can be found in The Library. Go to
http://www.ldwa.org.uk/library/hundredsurvival/hundredsurvival.htm